In general, conditioning air from an air conditioning unit is introduced to an air blowing port of an instrument panel through an air duct and blown into a passenger compartment. The air duct is connected to a case of the air conditioning unit and the instrument panel in manners shown in FIGS. 4A and 4B, for example.
According to the first manner shown in FIG. 4A, a tubular air duct 13 is first fastened to a case 2a of an air conditioning unit 2 by tapping screws 4. Then, the case 2a and the air duct 13 are mounted on a vehicle body B. In this condition, an instrument panel 1 is mounted on the vehicle body B so that an end 13a of the air duct 13 is connected to an air blowing port 1a of the instrument panel 1. Here, a sealing member 5 such as urethane packing is provided between the end 13a of the air duct 13 and the air blowing port 1a of the instrument panel 1 for sealing between them.
According to the second manner shown in FIG. 4B, the air conditioning unit 2 is previously mounted on the vehicle body B. The first end 13a of the air duct 13 is fastened to the air blowing port 1a of the instrument panel 1 by the tapping screws 4. In this condition, the instrument panel 1 is fixed to the vehicle body B with the air duct 13, so the second end 13b of the air duct 13 is located on and communicated with an air outlet port of the case 2a. The sealing member 5 is provided between the connecting portions of the second end 13b and the air outlet port of the case 2a. 
Since the sealing member 5 is required between the connecting portions of the air duct 13 and the case 2a, the numbers of parts and working steps increase. Further, this results in an increase of costs in the assembly.
Also, when the large instrument panel 1 is fixed, the air duct 13 is shielded by the instrument panel 1. Accordingly, it is difficult to check that the end of the air duct 13 is properly connected. This results in a deterioration of workability of fixing the instrument panel 1. With regard to the second manner, it is likely to be difficult to position the end 13b of the air duct 13 to the air outlet port of the case 2a because of size and shape variations of the air duct and the air conditioning unit 2.